Some automotive technologies require the use of a valve in the exhaust gas flow to aid in the control of emissions. For example, to meet some stringent emission requirements it has been proposed to include two catalytic converters in the vehicle, one of which is a warm-up converter and the other of which is a conventional converter. The warm-up converter operates before the engine is warmed up to improve the efficiency of the removal of undesired species in the exhaust gases until the conventional converter is warmed up and is sufficiently removing the undesirable species from the exhaust gases. Use of the warm-up converter becomes unnecessary once the conventional catalytic converter is warmed up. Depending on the warm-up converter structure, location or active catalyst formulation, continued use during operation of the vehicle may damage the converter or cause it to age prematurely.
To prevent damage to the warm-up converter, exhaust gases are rerouted after the warm-up period, preventing them from flowing into the warm converter. Thus, a valve must be provided in the exhaust system for valving exhaust gas flow, allowing the exhaust gas to flow into the warm-up converter during engine warm up operation and preventing exhaust gas from flowing into the warm-up converter after the warm up period. The valve must be capable of operating over a large temperature range, for example, from well below 0.degree. C. to over 1000.degree. C. Many existing valves simply are not robust enough to endure in the vehicle over such operating ranges. Other valves use ceramic structures that are durable but may be difficult to make and/or difficult to interface with the steel of the vehicle's exhaust system.
It is desirable that the valve be substantially sealed when closed to block exhaust flow from entering the start up catalytic converter and that the valve not allow escape of gas through its joints from the interior of the exhaust system.
One known type of butterfly valve shuts off gas flow by rotating the butterfly plate until the perimeter of the plate contacts the inner bore of the housing. With this design it is difficult to achieve a good seal over a large operating temperature range due to thermal expansion and contraction of the valve plate and of the valve housing. This design is further complicated by extremely tight dimensional tolerances, increasing valve cost. If tight tolerances are not maintained and/or the thermal expansion is not accounted for, the valve may not seal well when in the fully closed position or have other failings caused by the expansion and contraction of the valve plate over the extreme temperature range of operation.
Another known type of valve design provides two crescent shaped surfaces on the inner bore of the housing, one surface facing each direction, for the blade to seal against to stop flow past the blade. The thicknesses of the crescent surfaces start from nil and extend to maximum thickness at the part of the plate 90.degree. from the pivot axis and approaches zero again on the other side of the pivot axis. This type of valve also suffers from leakage when in the closed position due to thermal expansion and contraction of the valve plate and flow housing and due to dimensional tolerance and process variations, creating gaps between the valve plate and the housing along the narrower portions of the crescent-shaped surfaces.